I recently got the Smoothie board. Today when I was going to set it up, it worked for a while, I was able to move the motors and heat the bed (did not actually print anything). However, after experiencing some problems with the extruder I tried replacing it with another motor, upon turning it on the usb was not not detected. I have tried reformating the sd card and have even installed a new one with a fresh firmware and config file, removing everything from the board, but still no luck. Only the 3.3v led turns on (the four other leds seem to turn on for a period after ejecting the sd card and reinstalling it but promptly turn off). Also the main processor on the board gets rather hot (uncomfortable to touch for after a short time) when plugged in through usb. What is the most likely problem with this board and how can I fix it?

I'm sorry to say it sounds very much like you burned the microcontroller. It could be due to a short, or to electrostatic shock …
The first thing you want to do, is check there is nothing on the board that could cause a short.
Then you'll likely want to try replacing the microcontroller, if you have the skills to do so. A few folks on the forum have done this succesfully ( see the threads ). It's not fun, but it works.

Okay, is there any way that I could verify that it is the microcontroller? I have disconnected everything and have checked for shorts, so you are probably right but I would just like to double check before attempting to soldier. Also what is the most likely cause of this occuring, everything was properly pluged in and there did not appear to be anything that could cause a short on the board, although thinking back on it I do recall hearing one and immediately unplugging the board. Also do you happen to know the links to those threads on the forum? Thanks again for all your help:)

from the symptoms, it's clearly the microcontroller.
you want to check all of your wiring for shorts, so you make sure this doesn't happen to the new mcu. but it could also have been ESD or something like that …

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